A triangularly-arranged three-legged core type three-phase transformer is proposed in which three iron cores, in which unit blocks, whose lateral cross-section is parallelogram-shaped and in which magnetic steel plates of a predetermined width are laminated, are set face-to-face with one another and are joined at 60° angles and the outer tangent line thereof is substantially circular, are arranged at the vertices of an equilateral triangle and are made to stand side-by-side with respect to one another, and upper and lower ends of these three iron cores are respectively joined by yokes (Japanese Patent Application Laid-Open No. 9-232164).